The present invention relates to aqueous compositions containing solubilized or dispersed therein one or more useful surfactants which exhibit low solubility (or no solubility at all) in water, especially in the presence of electrolytes and/or pH agents. For purposes of this application, a substance is considered to be "solubilized" in water if the material is dissolved in the water or if it is uniformly dispersed or distributed therein, or emulsified therein, so as to exhibit the physical appearance and physical properties of a single-phase system (whether as an emulsion, an organic-based formula, or a water-based formula).
As is well known, surfactants can be used to perform a wide variety of useful purposes, ranging from cleaning and surface protection through deposition of coatings, fabric softening, foam stabilization, oil recovery, ore flotation, asphalt emulsification, achieving or enhancing rewetting effectiveness and penetrating power, and a large variety of other capabilities set forth hereinbelow. However, in many cases the ability to take advantage of such surfactants' useful properties is limited by the low solubility and/or dispersibility of many surfactants and surfactant blends in water.
There is also need in the marketplace for products with higher levels of concentration of surfactants or active ingredients, thus minimizing the amounts of water in the products. As the amount of water in the formulations is reduced, and as formulations add more (and more complex) ingredients, the fluidity and stability become more difficult to maintain. Often the surfactants become insoluble gels when diluted in water, or become hazy or even split into different phases. Surfactants often become insoluble in formulations where the concentration of inorganic salts is very high. In other surfactant formulations, maintaining the fluidity and dispersibility of the surfactant in water are serious problems which limit their use and application. Many surfactants are difficult to even disperse in water, requiring both hot water and long periods of mixing for dissolution into solution.
Hydrotropes or, more generally, coupling agents are added to surfactant formulations to increase the amount of the relatively water-insoluble surfactants that can be solubilized into the system. In most cases, they do not act as surfactants to lower surface tension but they often allow surfactants in the presence of salts or electrolytes to be added and subsequently dispersed into water at higher concentrations or at lower viscosities of the formulation than is otherwise achieved using only surfactant and water. These coupling agents assist surfactants by increasing the surfactant's solubility in water and its stability in the formulation, especially in the presence of salts, electrolytes and/or pH agents.
Hydrotropes or coupling agents generally contain short chained (C.sub.2 -C.sub.6) hydrophobes with more bulky hydrophilic group(s) such as hydroxyl and/or sulfonates making them completely water-soluble. They are normally added to stabilize formulations of surfactants, salts and water and to hold them in single-phase systems.
Materials that have been proposed for use as coupling agents include hexylene glycol, propylene glycol, dipropylene glycol, diethylene glycol, any of various lower alkoxy-capped glycols or polyglycols, particularly where the glycol is ethylene glycol or propylene glycol, such as ethylene glycol monobutyl ether, alcohols such as isopropanol and ethanol, and certain aryl sulfonates such as sodium naphthalene sulfonate and sodium xylene sulfonate, as well as some phosphate esters. However, despite the abilities of these water-soluble products as coupling agents that have been suggested for these materials, there remains a need and an interest in identifying coupling agents and systems containing such coupling agents which not only exhibit superior stability and superior ability to solubilize relatively water-insoluble agents but also improve formulation fluidity, dispersibility and product performance.
In addition, coupling agents that have improved permissible-exposure limits, higher flash points (over, for instance, isopropanol and ethanol), and lower odor (compared to, for example, butyl CELLOSOLVE.RTM. (a trademark for 2-butoxyethanol) or isopropanol) would have substantial importance to formulations and consumers. Other coupling agents such as sodium xylene sulfonate containing aromatic rings have come under environmental scrutiny in recent times.
The composition disclosed in U.S. Pat. No. 4,692,277 represents an attempt to incorporate certain diol solvents into hard surface cleaning formulations. The disclosure, however, is limited to liquid hard surface detergents/cleaners which contain at most 10% of a surfactant, and which must contain 1% to 30% of a detergency builder salt. The necessity of these components in the indicated amounts attests to the specific, limited nature of the teachings of this patent. More fundamentally, the disclosure of this patent was concerned solely with the solvent power of C.sub.6 -C.sub.16 diols as to their effect on soap scum removal when combined with both surfactants and salt builders, and completely fails to suggest or appreciate that it is possible, through selection of components according to the present invention, to employ certain diols and/or alkoxylates thereof so as to attain the solubilization of much higher amounts of less soluble surfactant(s) while retaining the desired monophasic state of the resulting composition. Thus, in turn, this patent thereby also fails to suggest any of the many end-use (especially monophasic) formulations that can be prepared embodying the compositions afforded by the present invention in combination with other hydrophobic surfactants, be they active ingredients or otherwise.